This application relates to image processing in digital camera, video, and other electronic digital image acquisition and/or display devices, and particularly to techniques of increasing the contrast of local image highlight details for such images.
Details in bright areas of a digital image, although they are often present in the acquired digital image data, may be hard to perceive, due to sensor non-linearities and limited system dynamic range. Consequently, such images could benefit from techniques of increasing the contrast of local image highlight details, while preserving the appearance of the overall image. This process is called Highlight Recovery (HR).
Optical sensors used in digital photography, video, and other imaging systems typically suffer from non-linear responses when rendering image regions under very bright, or very dim, lighting conditions. A unit increase of light intensity does not typically yield a unit increase in signal output from the sensor, particularly at the extreme ends of the sensor's operating range. In digital cameras, this problem is usually remedied at the dark end of the sensor's range by use of “digital lighting” algorithms, typified by Retinex-type approaches, that can enhance contrast in shadowy regions of the image. Although the problem of highlight information loss in bright areas can be addressed by the tailoring of such prior art techniques for this purpose, a search of prior art techniques indicates that a simple and straight forward approach to the recovery of highlight information in digital images, has yet to be specifically disclosed.
Prior art typically assumes that special methods and apparatuses need to be contained within any dynamic range compensation technique that aims to support both lighting and HR. HR is usually not seen in today's digital image enhancement applications, or consumer cameras, that provide a digital lighting feature. Therefore, there is substantial room for improvements in digital imaging systems by the inclusion of, or improvements in, highlight recovery.